The present invention relates to a method and apparatus for making alloys, and more particularly to a method and apparatus for solidification-controllable induction melting of alloy with a cold copper crucible.
Generally speaking, a cast alloy having a pore-free and solidification-controlled special microstructure is superior to a traditionally cast alloy ingot in its strength, toughness and surface property, and is therefore a necessary material in modern electronic, semiconductor, machinery, aviation and defense industries for making, for example, superalloy turbine blades. Such cast alloy may also be employed in the electronic industry for making, for example, target material, if it is possible to obtain solidification-controlled fine crystal grains. However, long-term research has found that there are more impurities in a crystal boundary, making the crystal boundary relatively weak and allowing quicker diffusion when it is subjected to force under high temperature. A crack often extends along a transverse crystal boundary that is perpendicular to the direction of an applied force. One way for the cast alloy to have an upgraded performance is to cause the crystal to grow in the direction of the applied force, so as to eliminate the transverse crystal boundary and impurities. This is an advantage provided by the so-called directional controlled solidification.
In the directional solidification of an alloy to obtain a directionally solidified structure for the alloy, it is important to select proper alloy properties and correct parameters for casting apparatus.
In conventional general apparatuses or methods for obtaining directionally solidified structure, such as the heat-generating agent process (EP process) taught by McLean M. et al (xe2x80x9cDirectionally solidified materials for high temperature servicexe2x80x9d, The Metals Society, 1983), the power reduction process (RD process) taught by VerSnyder F. L. et al (Modern Casting, 52(6): 68xcx9c75, 1967), and the high-rate solidification process (HRS process) taught by Higginbotham G J S et al (Mater. Sci. Technol., 2:442xcx9c459, 1986), the resultant alloys tend to have a microstructure showing relatively large difference at different areas and having uneven components, as in the case of Taiwanese Patent No. 415593 disclosing a system for measuring unidirectional solidification heat transmission in metal molds. Moreover, the conventional apparatuses or methods for obtaining a directionally solidified structure are usually expensive and have low productivity and reproducibility.
The currently available methods for melting and directional-controlled solidification of metals and non-metals, such as silicon, titanium, zirconium, etc., for manufacturing active alloys mainly include vacuum arc melting, vacuum induction melting, electron beam melting, plasma melting, etc. Among these methods, the vacuum arc melting method requires very high quality electrodes and raw materials, and needs additional alloy melting and casting to prepare the electrodes, which frequently have many shrinkage holes and impurities to adversely affect the quality of finished products manufactured through vacuum arc refining. As for the electron beam melting and the plasma melting, they have the disadvantages of requiring high vacuum and failing to remove gas impurities, as well as having an increased cost for maintaining the melting apparatus. A conventional induction-melting furnace has high melting efficiency. However, the problem of contamination of melting material by the refractory material of the crucible exists even in the melting of active metals, such as titanium alloys, with the vacuum induction-melting furnace. Before 1950""s, people always tried to use ceramic crucibles to melt active metals, such as aluminum, silicon, titanium, etc. However, serious chemical reaction tends to occur between the ceramic crucible and metal melt to contaminate the resultant alloy. Thus, it was almost impossible to obtain highly pure active metals. However, new technological developments and industrial demands in recent years have resulted in the use of cold copper crucible in place of the ceramic crucible to solve the contamination problem.
For example, U.S. Pat. Nos. 5,892,790, 5,563,904, 6,144,690, and 6,210,478 all disclose melting apparatus similar to the induction furnace and using cold crucibles having differently shaped slits. According to these earlier patents, eddy current is produced in the metal melt in the cold crucible through electromagnetic induction. Due to a resistance of the metal melt, Joule heat is generated to heat and melt the metal, which is further stirred and becomes suspended state under the effect of electromagnetic field. The vacuum induction melting process is actually a combination of the conventional induction melting techniques with vacuum techniques to simultaneously control two variables, namely, pressure and temperature. With the vacuum melting, a pressure difference in the crucible causes gases in the metals to diffuse to the liquid surface of molten metals and is therefore removed from the metals, enabling largely reduced gas amount in the resultant ingot. Meanwhile, impurity elements are separated from the melt due to heat convection and density difference to locate at the liquid surface of the molten metals, enabling a good purifying effect.
T. Nakajima et al. (U.S. Pat. No. 5,892,790) have conducted researches about the influences of local refining and cold crucible vacuum induction melting under ultra vacuum on the purity of Tixe2x80x94Al alloy. The research result indicates that the content of oxygen in the melt can be reduced to 85 ppm under an ultra vacuum of 10xe2x88x927 Pa when the cold crucible vacuum induction melting process is used to manufacture Tixe2x80x94Al alloy. In addition, supplying of argon gas in the melting would reduce the vaporization of aluminum but increase the content of oxygen in the melt materials. This problem may be somewhat alleviated by repeatedly highly vacuumizing the crucible and then supplying argon gas into the crucible for several times. On the other hand, while remelting via local refining enables reduction of content of oxygen to 13 ppm, the productivity thereof is low and the vaporization of aluminum is high to result in difficulties in controlling the alloy ingredients and mass production.
Kenji Abiko and Seiichi Takaki (Vacuum, Vol. 53, 1999, pp. 93-100), use cold crucible vacuum induction melting process to melt iron under an ultra vacuum of 7.5xc3x9710xe2x88x926 Pa. The result indicates the contents of carbon, nitrogen, oxygen, sulfur, and hydrogen all are lower than 10 ppm.
All the methods and apparatus disclosed in the above-mentioned patents and references require additional directional solidification control equipment and cooling water to obtain the directionally solidified cast structure. It is therefore desirable to develop a method and apparatus enabling direct solidification control after melting to form the directionally solidified structure for the melted metals, so as to eliminate drawbacks existed in the conventional melting processes and to reduce overall costs for melting alloys and controlling the solidification of ingot.
A primary object of the present invention is to provide a method and apparatus for solidification-controllable induction melting of alloy with cold copper crucible, so as to enable direct solidification control after melting to obtain directionally solidified structure for the melted metals at reduced overall costs for melting alloys and controlling the solidification of ingot.
The method for solidification-controllable induction melting of alloy with cold copper crucible according to the present invention includes the following steps:
a. Position an alloy material in a material zone of a cold copper crucible included in a vacuum induction furnace apparatus;
b. Vacuumize the material zone to a predetermined degree of vacuum, and supply an inert gas into the material zone to produce a predetermined pressure;
c. Repeat the step b for several times to rarefy unwanted gases in the material zone and to increase differential pressure of gases in the furnace apparatus;
d. Supply a current having a predetermined frequency during melting; and
e. Stop supplying of the current after a predetermined period of time.
The apparatus for solidification-controllable induction melting of alloy with cold copper crucible according to the present invention includes a vacuum pump, a vacuum meter, a crushed material feeder unit, an oil-pressure unit, an induction generator, and a crucible assembly. The crucible assembly includes a crucible having an internal metal material zone, and an induction coil provided on an outer surface of the crucible. The induction coil is wound from a copper tube that is coated with a silicon thermal-resistant insulating fiber jacket. The induction coil is internally provided with a cooling water circulation passage to communicate at two ends with external water inlet and water outlet made of copper tubes.